The purpose of the present study is to analyze the mechanisms regulating fundamental properties of motor nerve terminals and muscle membrane as these mechanisms recover during the reinnervation process following a crush of the muscle nerves in newborn and adult rats. These studies will be performed upon the extensor digitorum longus and the soleus muscles which are fast (twitch) and slow (tonic) contracting muscles, respectively. The objectives of this project are: (1) to compare and contrast the role of axoplasmic flow and muscle use upon the neural influences regulating muscle; (2) to evaluate the role of sensory input, via muscle spindles, in the neurotrophic regulation of muscle; (3) to quantitatively evaluate the action of drugs which enhance or diminish transmitter release from normal nerve terminals upon this process during progressive stages of nerve regeneration; (4) to determine the effect of neonatal and adult thyroidectomy upon the development and maintenance of fast and slow muscle types; (5) to evaluate the influence of axoplasmic flow, muscle use, sensory input, transmitter release, and thyroid hormone upon a ouabain-sensitive electrogenic pump intimately involved in the maintenance of a critical fraction of muscle membrane potential; (6) to determine the importance of the activity of such a pump to the development and maintenance of fast and slow muscle types.